A prior art accelerometer is described in U.S. Pat. No. 4,779,463. The servo accelerometer comprises a detachable case, suspension means for suspending the proof mass, placed in conductive tracks for supplying the current from the case to the proof mass means, said tracks being connected in series to form a compensation loop, comprising a differential angular-movement pickoff, a servo amplifier, and a differential torquer with movable coils.
In this device a proof mass paddle, four flexures of suspension means and a mounting frame are made of an integral wafer of the silicon monocrystal.
The disadvantages of said accelerometers are:
1) It is difficult to mount the proof mass within the device case.
2) There is a significant pickoff zero signal in stability, causing the accelerometer operation errors.
3) Carcasses on which the torque coils are wound are attached directly to the silicon base of the proof mass.
A second type of prior art accelerometer is described in U.S. Pat. No. 4,498,342. The servo accelerometer comprises a detachable case, suspension means for suspending the proof mass placed therein, and a gas damper. A differential pickoff, a preamplifier, a correction unit, a power amplifier and a differential torquer with movable coils are connected in series and form a compensation loop of said accelerometer. Here the proof mass base, suspension means and the mounting frame are made from the integral wafer of the silicon monocrystal.
In this accelerometer each stator of the torquer includes a permanent magnet, and each surface of the proof mass paddle has a force balancing coil (torquer coil) mounted on it. Current flowing through each coil produces a magnetic field that interacts with the permanent magnet of the associated stator, to produce a force applied to the proof mass. By controlling the electrical currents supplied to the coils, one can control the magnitude and direction of this force.
A differential pickoff of this device is implemented as a whetstone bridge whose arms include the tensoresistors. The tensoresistors are formed at the silicon flexures of elastic suspension means.
Movement of the proof mass with respect to the stators causes the differential resistance of the pickoff to change, which change can be used to determine a proof mass position.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensing axis results in rotation of the proof mass about the suspension axis with respect to the stators. The resulting differential resistance change caused by this movement of the proof mass is sensed by a feedback circuit. The feedback circuit responds by producing a current that, flows through the force balancing coils and produces a force that tends to return the proof mass to its neutral position. The value of the current required to return the proof mass in its neutral position allows to measure of acceleration along the sensing axis.
A compensation type accelerometer described above is the closest to the invention and therefore, is accepted by the authors as a prototype. The disadvantages of the prototype are:
1) Implementation of the pickoff as a tensoresistive bridge results in quite a high temperature drift of a zero signal of accelerometer pickoff. Since the device proof mass has the elastic suspension means, said zero signal drift causes a considerable error in operation of a compensation type accelerometer, which error is determined by the suspension means flexure stiffens.
2) The proof mass silicon wafer is not absolutely isolated from the influence of temperature strains of the accelerometer case, because the case is made of a material having the thermal expansion factor that differs from that of silicon.
3) The temperature strains of reducing bushes for attaching the torquer coils affect deformations of the proof mass paddle, for the thermal expansion factor of bushes is not equal to that of the proof mass paddle.